The invention relates to a method for producing in particular structural or chassis components for a motor vehicle by hot or warm forming, wherein at least a first region of a sheet metal blank is heated in a heating station from a starting temperature to a target temperature and subsequently the hot blank is transferred into a cooled pressing tool where it is formed and press-hardened. The invention further relates to a heating station for use or application in the method.
For producing in particular structural and chassis components for vehicles, oftentimes sheet metal blanks are cut from a strip material, heated in a continuous furnace and subsequently hot formed into a formed component and press-hardened in a cooled pressing tool. This method is also known as direct hot forming. Besides this, indirect hot forming is common where a sheet metal blank is first cold formed and then heated before it is finally formed or configured in a cooled pressing tool and press-hardened. The heating of the sheet metal blank typically occurs in continuous furnaces, wherein indirect heating occurs via the heated air through heat radiation and convection. In a continuous furnace the sheet metal blank is to be heated under defined conditions. Depending on whether uncoated or coated starting materials are used, the heating parameters have to be adjusted.
DE 10 2010 004 081 B3 discloses a direct and indirect hot forming of steel for producing components of different ductility, wherein the blank or the pre-formed component is heated to a temperature smaller or equal to Ac1 and only certain regions are heated by oil or gas burners to greater or equal to Ac3 and the blank or the pre-formed component is subsequently press-hardened.
WO 93/20248 A1 discloses a multistep heating of thin metal parts in continuous furnaces; wherein heating is carried out to a pre-heating temperature under atmosphere up to 850° C. and then to final temperature in reduced (O2-free) atmosphere with upper and lower burners.
A disadvantage hereby is the long time required for the heating and as the case may be for the alloying of the pre-coating and the high space requirement of the continuous furnaces resulting therefrom in order to maintain predetermined cycle times. As a result of the long heating time the re-crystallization or austenization of the starting material occurs in such a manner that a relatively coarse-grained microstructure is established, which is frozen into martensite in the subsequent press-hardening. This microstructure is disadvantageous with regard to the ductility of the finished component, which may be particularly important for crash relevant components.
DE 10 2006 005 063 A1 discloses a method for heat treating uncoated steel products in which a continuous furnace is equipped with burners whose flames are brought into direct contact with the steel products in order to enable a fast heating and on the other hand a pre-oxidation of the surface, which are required for subsequent coating steps. Different temperature zones and the oxygen content in the region of the strip surface are adjusted via the air number Lamda (supplied amount of oxygen relative to the amount of oxygen reacted during combustion) as burner parameter.